I. Field of the Invention
This invention relates to electrically conductive, highly visible balls, and particularly to tennis balls used in systems comprising circuits formed by conductive elements on or adjacent to the court playing areas and capable of activating visual and audible indicators when closed by such a ball dropping on a line which is part of the court, or dropping outside the playing area. While the present invention may be employed in the manufacture of different types of balls for other games, references herein will be made primarily to conventional tennis balls, although such reference is meant to include balls for other games as well.
II. Description of the Prior Art
The prior art discloses a number of efforts to provide court and all game indicating systems, such as for tennis, in which players, spectators and umpires are able to determine accurately by visual or audio means, or both, whether or not a ball drops, i.e. touches the ground, within or outside the designated playing area. Such a system is disclosed in Swedish Pat. No. 206,864 to Johansson. Other, improved systems are described in applicant's U.S. Pat. Nos. 3,883,860 and 4,092,634. In these systems, parts of the playing areas, such as the lines, and the areas outside the lines defining the in-bounds playing area, have thereon electric leads, which leads are part of a circuit which is closed by an electrically conductive ball coming in contact with two or more such electric leads. However, past efforts to make balls sufficiently electrically conductive to close such circuits have had the serious disadvantages of either resulting in balls with unacceptable playing characteristics or with poor in-flight visibility, due to discoloration caused by the dark color of the substances used to impart electric conductivity to the balls.
An electrically conductive ball is disclosed by Supran, U.S. Pat. No. 3,854,719; this ball has steel wire or staple sliver steel uniformly woven into the cover cloth panels. However, steel or any other metal fibers, due to their stiffness and high specific gravity, impart highly altered and completely unacceptable playing characteristics to tennis balls. Supran also mentions spraying or otherwise applying electrically conductive paint onto the outer surface of the ball. Applicant is not aware of any conductive paint which, when sprayed onto the entire surface of a ball, will not excessively discolor the ball, i.e. diminish its visibility in flight, nor change its playing characteristics to an unacceptable degree. The use of conductive substances, such as carbon black or carbon fibers to make textile products electrically conductive for purposes of dissipating static electric charges has been suggested by Hotta et al. U.S. Pat. No. 3,746,573 and by Fujiwara et al. U.S. Pat. No. 3,708,335. However, the recommended amounts of such conductive materials are very small, as relatively large amounts would discolor the products to an unacceptable degree; the conductivity of textile products thus produced is inadequate for the contemplated purpose.
It therefore is an object of this invention to provide balls for use in ball game indicating systems.
It also is an object to provide balls for use in court and ball game indicating systems, such as are described in applicant's U.S. Pat. No. 4,092,634.
It is another object of the present invention to provide electrically conductive balls, such as tennis balls having normal playing characteristics.
It is yet another object to provide electrically conductive balls, such as tennis balls, comprising on their playing surface conductive substances darker in color than the rest of the ball surface, said balls nevertheless having unimpaired visibility in flight.
It also is an object to change the disadvantage hitherto associated with the use of discoloring, conductive substances to the advantage gained by improving ball visibility by using conductive discolorants in the manner set forth.